This invention relates to thread-forming screws and more particularly to a novel thread-forming screw having a circular threaded main body portion for maximum holding power and a novel lobular tapered pilot portion adapted for forming threads in a workpiece with minimum torque, and to apparatus and a method for making the same.
Self-tapping or thread-forming screws in many different variations are known to the art. In general, these screws fall into two types, viz., those in which the thread-forming portion cuts away a portion of the workpiece to produce the thread and those which form the thread by causing the metal or other material in the workpiece to flow without removing any material. Of the two types, thread-forming screws are generally preferred over thread-cutting screws because the former work harden the mating hole for improved strength; they produce no chips or other residue which may create problems; they usually provide better vibration resistance; and, they require fewer operations in their manufacture. The screw of the invention falls in the thread-forming category.
Among the thread-forming screw designs which have been proposed, as shown in U.S. Pat. Nos. 3,083,609; 3,469,491; and 3,472,119, and British Pat. Nos. 582,306; 669,157; and 925,135, are those in which the screw is made from a cylindrical blank having a tapering point, the blank having formed thereon a continuous thread which has a constant minor diameter. Because of the tapering point, the threads in the pilot portion of the screw are not fully formed and have dull unfinished crests. In a variation of such screws, the tapered point of the screw blank is flattened to produce three or more facets on the tapered point before the threads are formed. The flattened facets are intended to reduce circumferential friction by lowering the extent of metal-to-metal contact between the screw and the workpiece. Such screws require relatively high tapping torque since both the crests of the tapping threads and, in the faceted variation, also the leading edges or ramps thereof are dull and therefore inefficient for thread formation.
Another known form of thread-forming screw, as exemplified by U.S. Pat. Nos. 3,195,156; 3,249,152; and 3,426,820, uses a blank which is non-circular, e.g., triangular, throughout its length. After a continuous thread is formed on such a blank, there is produced a screw having threaded lobes which serve to form the threads in the workpiece. Since the thread is sharp and continuous throughout its length and because contact with the workpiece is restricted primarily to the vicinity of the lobes, the tapping torque required with such screws is desirably low. By the same token, however, once the thread is tapped in the workpiece and the screw is fastened in position, its holding power is diminished because of the fact that full engagement of the threads of the screw and the workpiece occurs only in the vicinity of the lobes. Accordingly, the holding power of such a lobular screw is less than that which can be achieved with a circular threaded screw. In addition, the lobular configuration of the blank requires additional steps in its manufacture which tend to increase the cost.
Still another variation of known thread-forming screws, as shown in U.S. Pat. Nos. 3,246,556 and 3,263,473, has a shank which is circular in its main body portion but lobular, e.g., triangular, in the pilot portion, the screw having a thread which is continuous in both the main body and pilot portions. While screws of this type can have a desirable combination of low tapping torque and high holding power, the manufacture of such screws requires additional steps, e.g., the formation of a triangular pilot portion on a circular main shank, or the use of special rolling dies having an undulating surface, both of which increase the cost of manufacture of the screw.